In the manufacturing of integrated circuits, after the individual devices such as the transistors have been fabricated on the semiconductor substrate, they must be interconnected in order to perform the desired circuit functions. This interconnection process is generally called “metallization” and is performed using a number of different photolithographic, deposition, and removal techniques.
A commonly used process for forming interconnect structures is referred to as a “damascene” process. In a typical damascene process, dielectric layers are deposited over the devices, followed by the formation of openings in the dielectric layers. Conductive materials are then deposited in the openings. A polish process is used to planarize the conductive materials with the surfaces of the respective dielectric layers causing the conductive materials to be “inlaid” in the respective dielectric layers.
Copper is typically used for the damascene processes. Copper has low resistivity, thus the RC delay caused by the resistance in the interconnect structure is low. However, with the scaling down of the integrated circuits, the dimensions of copper interconnects are also accordingly scaled down. When the dimensions of the copper interconnects approach the mean free path of electrons, the resistivity of the interconnect structure significantly increases. As a result, the RC delay from the interconnect structure significantly increases.
Various methods have been explored to reduce the resistivities of the interconnect structures. For example, barrier layers, which are used to prevent copper from diffusing into neighboring low-k dielectric layers, typically have high resistivities. Thus, methods for forming thinner barrier layers were explored. Accordingly, copper lines will have greater sizes even though the sizes of the trenches, in which barrier layers and copper lines are formed, are not reduced. Also, atomic layer deposition (ALD) is used to form diffusion barrier layers for reducing the resistivity of copper lines formed thereon. These methods, however, incur additional problems. For example, thinner barrier layers may have reduced ability to prevent copper diffusion. In addition, electro-migration and/or stress-migration problems may arise. The use of ALD may cause the precursors to penetrate into the pores of low-k dielectric layer. Since the precursors include metals, the characteristics of the low-k dielectrics are adversely affected. New methods are thus needed for reducing the resistivities of copper lines while at the same time overcoming the deficiencies of the prior art.